In the past few years, a considerable number of case reports describing predominantly gastrointestinal, urinogenital, and ophthalmic manifestations of microsporidial infections in AIDS patients have been published. Although anecdotal evidence of the therapeutic efficacy of a number of antimicrobial agents has appeared in the literature, there has been little or no in vitro data to substantiate these claims. Of principal species of microsporidia that are human pathogens, all but Enterocytozoon bienusii, have been propagated in tissue culture system, although there seems to be little consensus about which culture techniques yield the best results. The initial aim of this project is to develop reliable methods for propagating microsporidia in vitro, including, hopefully, E. bienusii. Once this objective is achieved, we intend to screen potentially useful antimicrobial agents to determine which, if any, have deleterious effects on microsporidial replication, and if this inhibition is species specific. Compounds will also be examined for their ability to prevent infection of cell culture monolayer by microsporidia. The ultimate goal of this study is to be able to correlate in vitro susceptibility results with in vivo therapeutic responses in order to help document the effectiveness, or lack thereof, of different pharmacologic modalities for treating microsporidial disease.